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Signalls the ending of a conversation. Usual follows a distinct point made. k=okay thanx=thankyou bai=goodbye. Kthanxbai replaces the need for "thats the end of it".
Msn Conversation:

Jeremy: Im so much cooler than you.
Andrew: No your not i am much cooler that you idiot.
Jeremy: how rude
Andrew: Just shut up you noob im cooler kthanxbai

*Jeremy has been blocked by Andrew*
by Andyrooroo June 07, 2006